The invention relates generally to power assisted braking systems for vehicles and more particularly, concerns a method and apparatus for controlling the brake system of a vehicle having frictional duty brakes and an electrically controlled driveline retarder brake adjusted as a function of the driver""s braking command.
Conventional retarder brake systems, also known as eddy current braking systems, comprise a portion (stator) that is fixed to the chassis of the vehicle and that includes inductor windings, and a moving portion (rotor) including an armature, which is coupled to a rotary element of the vehicle, generally the transmission shaft. The armature is typically a body of ferro-magnetic material which, when moving relative to excited windings has electrical currents known as xe2x80x9ceddyxe2x80x9d currents induced therein. Because the resistivity of the armature, these eddy currents cause energy to be dissipated, resulting in the rotor, and thus the vehicle driveshaft, being slowed down. The energy is dissipated typically in the form of heat, and the rotor is commonly given a finned configuration suitable for disposing of the heat.
Most retarder brake systems are manually operated wherein the driver of the vehicle can actuate a multi-position control lever to obtain a braking effect on the vehicle with a torque that varies depending on the position selected for the lever. This variability is obtained by a set of relays each serving to excite one of the windings, with the number of relays in the closed-circuit position depending on the position of the lever. Thus, the driveline mounted retarder brake assists the existing frictional brake system in the braking the vehicle.
Most retarder brake systems are manually operated. Thus, there exists a need for an automated control method for actuating a vehicle driveline retarder to improve vehicle braking performance.
It is therefore an object of the present invention to provide a control method for a vehicle electrical power assisted brake system which includes a driveline mounted retarder brake and frictional duty brake system.
According to the present invention, the foregoing and other objects are attained by a method of controlling a retarder brake system of a vehicle. The method comprises the steps of determining an operator braking input indicative of a desired vehicle deceleration and generating a desired retarder torque value (DT) corresponding to the desired vehicle deceleration value. The method further includes determining a wheel speed value for each wheel of the vehicle and generating a front-to-rear wheel slip (FRS) value indicative of rear wheel lockup as a function of the wheel speed values. A retarder torque reduction (TR) value is also generated as a function of the FRS value. A desired retarder torque command (TC) is then generated as a function of the DT and TR values. The method also includes the step of controlling the retarder as a function of the retarder torque command to achieve the desired vehicle deceleration. Thus, the TR value reduced the retarding torque which would otherwise be applied in response to the driver input, to prevent driven wheel lockup.
In another aspect of the present invention, the driver braking input includes a selectable vehicle braking characteristic in addition to the brake pedal input. Thus, for a given amount of brake pedal treadle, the operator can select the vehicle braking response by increasing or decreasing the retarder brake activation. In another aspect of the invention, the vehicle braking characteristic is nonlinear wherein, for increasing brake pedal treadle, the retarder brake provide increasing brake torque to the vehicle driveshaft. In this way, the vehicle operator can control the braking feel of the vehicle by overning the retarder brake actuation in response to brake pedal treadle. Further aspects of the present method incorporate traction control and activation of a limited slip differential.
An advantage of the present invention is that it extends the life of the frictional duty brakes by providing additional braking force via the retarder brake. Another advantage is improved brake pedal effort in that high braking torque at the wheels is achieved without requiring high pedal force and long pedal travel from the vehicle operator. Another advantage of the present invention is improved deceleration perception in that the retarder brake generates braking force at the rear wheels quicker than the frictional duty brake system thereby decelerating the vehicle with minimum pitch.
Another advantage of the present invention is an improved anti-lock brake strategy in that the brake retarder can be used to reduce braking torque when rear wheel lockup appears imminent. Other objects and advantages of the invention will become apparent upon reading the following detailed description of and appended claims, and upon reference to the accompanying drawings.